Field of Invention
The present invention relates to a composite membrane and distillation method and, in particular, to a composite membrane adapted for a vacuum membrane distillation device and a vacuum membrane distillation method using such a composite membrane.
Related Art
Lack of water resource is currently a worldwide problem. To solve this problem, numerous techniques about wastewater recycling and seawater desalination have been developed. Among these techniques, the distillation process including multi-stage flash distillation (MSF), multiple-effect distillation (MED) and vapor-compression (VC) and the membrane process are specially developed. Advantages of the distillation process include the high energy utilization rate and the capability of using waste heat generated by production lines of factories as energy. However, it still has a disadvantage of huge equipment. The membrane process using a semipermeable membrane may efficiently remove the impurities from the water, but it requires huge energy to overcome the osmotic pressure resulting in that the water can pass through the semipermeable membrane. Therefore, the cost of electricity is relatively high.
In order to overcome the disadvantages of the distillation process and the membrane process described above, the membrane distillation (MD) combining the distillation process and the membrane process has been developed. Its equipment is relatively small and capable of using solar energy or waste heat as energy, and it also maintains fine purification result.
In addition, the membrane distillation process can be classified into direct contact membrane distillation (DCMD), air-gap membrane distillation (AGMD), sweeping-gas membrane distillation (SGMD), and vacuum membrane distillation (VMD). Among these processes the vacuum membrane distillation has relatively low mass transfer resistance and excellent water flux.
However, the vacuum membrane distillation still has some problems. Because the basic principle of the membrane distillation process to separate the solute from the solvent is that a liquid/gas interface needs to reach the liquid vapor equilibrium, the membrane should not be wetted. Therefore, the pore diameter of the membrane should be small enough and the membrane should be highly hydrophobic, so that higher surface tension exists between the membrane and liquid. Hence, material and hydrophobicity of the membrane both influence the efficiency of the vacuum membrane distillation process.
Therefore, it is an important subject in the field to provide a membrane adapted for a vacuum membrane distillation device and having high hydrophobicity.